One of the Scottish National Party’s (SNP) biggest backers has warned that an independent Scotland could harm business. The Stagecoach group, which run buses and trains across the UK, warned in its annual report that independence would be a “risk” and “uncertainty” for the group, despite boss Sir Brian Souter being a major donor to the SNP.

Only last month, Sir Brian pledged £1m to the SNP, having previously given £100,000 to campaign group Christians for Independence.

Despite this, his company says: “Regulatory changes and availability of public funding – there is a risk changes to the regulatory environment or changes to the availability of public funding could affect the group’s prospects.

“Such changes may arise as a result of the outcomes of the September 2014 referendum on Scottish independence and/or the 2015 UK general election.”

Pro-Union supporters seized on the comments as proof that even independence supporters see breaking up the UK as a huge risk. Labour’s Ian Murray, MP for Edinburgh South, said: “When even Brian Souter, who has bankrolled the Nationalists for years, thinks separation is a big risk, you know Alex Salmond has a problem. This intervention drives a coach and horses through the Nationalists’ case for separation.

Murray added: “Being part of the UK single market works well for firms like Stagecoach. Scottish employers have access to a market of 63 million people across the UK, rather than five million people in Scotland. Where is the sense in putting up barriers between Scottish firms and their customers based elsewhere in the UK?”

Jackson Carlaw, deputy leader of the Scottish Conservatives, said: “Surely it’s ironic for Brian Souter to wager an each way bet, mouthing off and bankrolling Yes Scotland on the one hand, despite it seems, his views as the public face of Stagecoach being far from representative of the very company he has built from scratch?

“Scotland and Stagecoach realise independence is a one-way ticket to a poorer nation.”

The SNP played down the intervention, however. A spokesman said: “Investment in Scotland is booming in the run-up to the referendum, underlined by the Ernst & Young attractiveness survey showing foreign direct investment in Scotland last year reached its highest level for 16 years and Scotland retained its position as the top UK location for inward investment outside London.

“An independent Scotland will be a hugely attractive place to do business and invest in.”